The invention relates to formed bodies consisting of glass, especially glass panes, provided with electric strip conductors having a colored appearance of the electric circuits when viewed through the glass. In another aspect, the present invention relates to a method for production of the above formed bodies in which a colored layer and a conductive paste are applied in succession onto the optionally enamelled form body and the object obtained in this manner is subjected to a heat treatment.
The electrically conductive circuits or tracks on the rear windows of automobiles, antennas integrated into windshields and circuits for other purposes are obtained by means of the application of a paste based on noble metals, preferably silver, onto a formed body of glass and subsequently firing it in. After the firing, it can be determined upon looking through the glass that the color of the circuit varies from bright yellow to dark brown as a function of the state of the surface of the glass and of the firing temperature.
Various methods have been used with the goal of preventing this lack of homogeneity of color: Application onto the "tin" side of the float glass, adaptation of the firing temperature, and addition of pigments to the noble- metal pastes. None of these methods resulted in satisfactory results either as regards the appearance or on account of too great a change of the electric and mechanical properties; such as weldability of the tracks.
In order to avoid the phenomenon of the discoloration of the glass or of the enamel, which is caused by the thermo-activated migration of Ag.sup.+ from the layer of the circuit into the glass or enamel, thus adversely affecting the appearance of the glass pane, an attempt was made, to the extent possible, to coat the silver-based circuit by applying the silver pastes onto highly pigmented glass enamels and/or ones containing compounds capable of blocking the Ag.sup.+ migration.
U.S. Pat. No. 4,837,383 teaches that the migration of silver ions can be prevented by using an enamel consisting of a glass flux or paste based on a glass frit, and as pigments, elementary aluminum and/or lithium oxide as well as a vehicle or medium for the paste. Patent application EP-A 0,377,062 claims the improvement of the covering power of glass enamel over silver due to the addition of silicon, boron, carbon, lead and/or silver in their elementary form before making up the paste.
Such additives are not completely satisfactory for obvious reasons of cost (silver), on account of the special processing conditions conditioned by the addition of strongly reducing elements such as silicon or boron, for reasons of stability vis-a-vis acids (lithium) and of toxicity (lead). In addition, in spite of the aesthetic aspect which a complete coating of the silver circuit by a glass enamel would represent, the colors of these glass enamels are limited to a few black and gray tones.